Previous work on the role of Immune Complexes in the pathogenesis of Systemic Lupus Erythematosus (SLE), Rheumatoid Arthritis (RA) and related disorders will be continued. In studies on SLE primary emphasis will be placed on the isolation and characterization of undetermined types of complexes and certain low molecular weight anti complementary substances that are detected by interaction with the Clq component of complement. In addition to studies on their role in the disease process, clues to possible etiologic agents will be sought in their composition. A second main objective will be to study the role of complement deficiency states and sex hormones as possible predisposing factors in the disease. In studies on RA the primary goal will be to determine the role in the disease of rheumatoid factors that cross react with DNA-nucleoprotein. The role of immune complexes in mixed cryoglobulinemia and mixed connective tissue disease will also be explored.